The Enlarge/Reduce spell can make a creature one size category bigger.

Considering that two spell effects of the same nature cannot stack at the same place and time (you can't cast two Enlarges on the same Medium target to make it Huge), is there another way (spell, item, etc.) to increase a target's size category that could be stacked with the Enlarge effect in order to increase the target's size category twice ?

\$\begingroup\$I think, contrasting with keithcurtis' answer, that it'd be worth pointing out not only that you're not casting enlarge, but also you're not even gaining the effect of enlarge. Your size is just changing, so there is no question of effects stacking.\$\endgroup\$
– nitsua60Dec 8 '17 at 13:44

\$\begingroup\$@nitsua60 Fair point, I'm still new to tabletop RPGs and their rulings, just wanted to do one's bit.\$\endgroup\$
– AntiDrondertDec 8 '17 at 13:56

Enlarge would stack with the spell polymorph or the druid's Wild Shape feature, which allow you to assume a different size category, based on the shape. If you want to remain the same creature type, I don't think you can.

Using a Potion of Growth might require a DM call, since it says it duplicates the effect of the enlarge/reduce spell, but is not actually the spell itself. I think most DMs would disallow the combination, but since the potion is a consumable, DM-controlled resource, it's unlikely to become unbalancing.

Starting at 14th level, you gain the ability to channel the souls of your ancestors into your physical form. When you start casting a sorcerer spell on your turn and expend a spell slot, youcan increase your size by one category—from Medium to Large, for example.

You can either cast Enlarge on top of that, or wait until level 18, when you can use it twice per short rest, and are explicitly permitted to stack it on itself.

In addition, you can now use Rage of Fallen Ostoria twice between rests, but no more than once on a turn. If you use that feature while
under its effects, its increases to your size, hit points, reach, and walking speed are cumulative.

For a Non-Player Character Duergar

Monster Duergar have a once per short rest ability to gain similar effects to the enlarge part of the enlarge/reduce spell (it doubles weapon damage dice rather than adds 1d4 and requires no concentration, so usually superior). They become a large creature for the duration. Because this is a magical ability rather than a spell it can stack with the enlarge/reduce spell. And voila, you have a huge Dwarf.

The player race Duergar casts it as a spell, however, so unless you have a friendly NPC party member with the creature stats rather than player race stats there is probably not a time when you want to do this.